This invention relates to radio communication apparatus, and more particularly apparatus for controlling transmission output power utilized in a transmitter of such mobile radio communication apparatus as a motor car telephone system or the like.
Various apparatus of the type referred to above have been proposed. Usually, a method has been used wherein a portion of the transmission output power is detected, the detected portion is converted into a DC voltage proportional thereto, the DC voltage is compared with a reference voltage corresponding to a predetermined set output, and the input level or gain of a power amplifier is controlled by the output of the comparator by using a feedback control.
Typical prior art apparatus for controlling transmission power will be discussed later with reference to the accompanying drawing. Such apparatus are now being used extensively in radio communication apparatus because the apparatus is not influenced by ambient condition of the transmission output power and because it can be fabricated at a low cost.
However, when such apparatus is used for a mobile radio communication apparatus for use in a motor car telephone system, for example, there are the following defects.
More particularly, the gain of a B or C class amplifier utilized as a power amplifier of a mobile radio communication apparatus is caused to vary by the output power so that the gain of a negative feedback loop utilized to control the output power of the power amplifier also varies.
To discuss this problem in detail, the transmission output power of a typical radio communication apparatus is fixed and will not increased or decreased, but the transmission output power of a mobile radio communication apparatus utilized in a motor car telephone system is increased or decreased in accordance with the electromagnetic wave propagation state between a base station and a mobile station including a mobile radio communication apparatus, the base station and the mobile station constituting a communication system, for the purpose of smoothly operating the system. Although different depending upon the type of the communication system, the transmission output power is usually increased or decreased by certain decibels (dB) or varied stepwise.
The transmitting output power control apparatus is required to be constructed such that it should have a characteristic that can suppress or limit the width of the output variation and an output response characteristic which does not vary. But the prior art transmission output control apparatus could not satisfy these requirements.
In the prior art transmission output power control apparatus, since the gain of a feedback loop is caused to vary due to the transmission output power, the characteristic for limiting the width of output variation and the output response characteristic vary in accordance with the level of the transmission output power which is a set output. Furthermore, unless sufficient margin is assured for the loop phase characteristic for a high transmission output power, the loop gain increases when the transmission output power is set to a low level so that the feedback loop would have a positive feedback, thus causing the system unstable.